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| power management 1 www.semtech.com SC2446A dual-phase, single or dual output synchronous step-down controller description revision: november 9, 2005 features typical application circuit the SC2446A is a high-frequency dual synchronous step- down switching power supply controller. it provides out- of-phase output gate signals. the SC2446A operates in synchronous continuous-conduction mode. both phases are capable of maintaining regulation with sourcing or sinking load currents, making the SC2446A suitable for generating both v ddq and the tracking v tt for ddr appli- cations. the SC2446A employs fixed frequency peak current- mode control for the ease of frequency compensation and fast transient response. the dual-phase step-down controllers of the SC2446A can be configured to provide two individually controlled and regulated outputs or a single output with shared current in each phase. the step-down controllers oper- ate from an input of at least 4.7v and are capable of regulating outputs as low as 0.5v the step-down controllers in the SC2446A have the pro- vision to sense inductor r dc voltage drop for current-mode control. this sensing scheme eliminates the need of the current-sense resistor and is more noise-immune than direct sensing of the high-side or the low-side mosfet voltage. precise current-sensing with sense resistor is optional. individual soft-start and overload shutdown timer is in- cluded in each step-down controller. the SC2446A imple- ments hiccup overload protection. in two-phase single- output configuration, the master timer controls the soft- start and overload shutdown functions of both control- lers. ? ? ? ? ? 2-phase synchronous continuous conduction mode for high efficiency step-down converters ? ? ? ? ? out of phase operation for low input current ripples ? ? ? ? ? output source and sink currents ? ? ? ? ? fixed frequency peak current-mode control ? ? ? ? ? 50mv/-75mv maximum current sense voltage ? ? ? ? ? inductive current-sensing for low-cost applications ? ? ? ? ? optional resistor current-sensing for precise cur- rent-limit ? ? ? ? ? dual outputs or 2-phase single output operation ? ? ? ? ? excellent current sharing between individual phases ? ? ? ? ? wide input voltage range: 4.7v to 16v ? ? ? ? ? individual soft-start, overload shutdown and enable ? ? ? ? ? duty cycle up to 88% ? ? ? ? ? 0.5v feedback voltage for low-voltage outputs ? ? ? ? ? external reference input for ddr applications ? ? ? ? ? programmable frequency up to 1mhz per phase ? ? ? ? ? external synchronization ? ? ? ? ? industrial temperature range ? ? ? ? ? 28-lead tssop lead free package. this product is fully weee and rohs compliant applications ? ? ? ? ? telecommunication power supplies ? ? ? ? ? ddr memory power supplies ? ? ? ? ? graphic power supplies ? ? ? ? ? servers and base stations figure 1 l5 vo1 gnd r13 r28 r29 c40 c29 13 9 16 11 1 10 28 vddc vssc vi cb vddo vo vsso u9 i nt egrat ed mosfet/ dri ve r vin (12v) vo2 gnd r45 vo2 c23 rcs-5 c45 r14 rcs-6 c62 r46 r47 vin ref out (0. 5v) ref out (0. 5v) ref out (0. 5v) c63 r48 c64 r49 c65 r50 r51 c67 c68 r52 0 13 9 16 11 1 10 28 vddc vssc vi cb vddo vo vsso u10 i nt egrat ed mosfet/ dri ve r tp11 r53 0 r55 c6 c7 vo1 c70 l6 refout 9 pgnd 22 gdh 2 20 bst2 19 in1- 4 vpn 1 27 sy nc 6 cs1+ 1 pvcc 23 comp1 5 ss1/en1 28 cs1- 2 ss2/en2 15 comp2 11 in2- 12 re f in 10 cs2- 13 cs2+ 14 vpn 2 18 vin 2 17 bst1 26 gdh 1 25 gd l 1 24 gd l 2 21 re f 8 rosc 3 agnd 7 avcc 16 u3 SC2446A c71 c72 c73 c74 ving nd c75
2 ? 2005 semtech corp. www.semtech.com power management SC2446A absolute maximum rating exceeding the specifications below may result in permanent damage to the device, or device malfunction. operation outside of th e parameters specified in the electrical characteristics section is not implied. exposure to absolute maximum rated conditions for extended periods of time may affect device reliability. electrical characteristics r e t e m a r a pl o b m y ss n o i t i d n o cn i mp y tx a ms t i n u t u o k c o l e g a t l o v r e d n u d l o h s e r h t t r a t s c c v ac c v a h t c c v ag n i s a e r c n i5 . 47 . 4v s i s e r e t s y h t r a t s c c v ac c v a t s y h 2 . 0v t n e r r u c g n i t a r e p o c c v ai c c v 2 1 = c c v a85 1a m o l v u n i t n e r r u c t n e c s e i u q c c v ac c v a = c c v a h t v 2 . 0 -5 . 2a m r e i f i l p m a r o r r e 1 l e n n a h c e g n a r e g a t l o v e d o m - n o m m o c t u p n i) 1 e t o n (03v e g n a r e g a t l o v t u p n i g n i t r e v n i) 1 e t o n (0c c v av e g a t l o v t e s f f o t u p n ic 0 7 ~ 013 v m t n e r r u c s a i b t u p n i g n i t r e v n i - n o ni f e r 0 0 1 -0 5 2 -a n t n e r r u c s a i b t u p n i g n i t r e v n ii - 1 n i 0 0 1 -0 5 2 -a n e c n a t c u d n o c s n a r t r e i f i l p m ag 1 m 0 6 2 ? 1 ? n i a g p o o l - n e p o r e i f i l p m aa 1 l o 5 6d h t d i w d n a b n i a g y t i n u r e i f i l p m a 5z h m d l o h s e r h t g n i h c t i w s 1 p m o c m u m i n i mv + 1 s c v = - 1 s c 0 = v 1 s s g n i s a e r c n i 2 . 2v r e t e m a r a p l o b m y ss g n i t a r m u m i x a ms t i n u e g a t l o v y l p p u s c c v a6 1 o t 3 . 0 -v s e g a t l o v 2 l d g , 1 l d g , 2 h d g , 1 h d g s t u p t u o e t a gv 1 h d g v , 2 h d g v , 1 l d g v , 2 l d g 6 o t 3 . 0 -v s e g a t l o v - 2 n i , - 1 n iv - 1 n i v , - 2 n i 3 . 0 + c c v a o t 3 . 0 -v f e r t u o s e g a t l o vv f e r v , t u o f e r 6 o t 3 . 0 -v f e r , f e r n i e g a t l o vv n i f e r 3 . 0 + c c v a o t 3 . 0 -v s e g a t l o v 2 p m o c , 1 p m o cv , 1 p m o c v 2 p m o c 3 . 0 + c c v a o t 3 . 0 -v s e g a t l o v - 2 s c d n a + 2 s c , - 1 s c , + 1 s cv , + 1 s c v , - 1 s c v , + 2 s c v - 2 s c 3 . 0 + c c v a o t 3 . 0 -v e g a t l o v c n y sv c n y s 3 . 0 + c c v a o t 3 . 0 -v s e g a t l o v 2 n e / 2 s s d n a 1 n e / 1 s sv , 1 s s v 2 s s 6 o t 3 . 0 -v e g n a r e r u t a r e p m e t t n e i b m a t a 5 2 1 o t 0 4 -c ) 8 2 - p o s s t ( e s a c o t n o i t c n u j e c n a t s i s e r l a m r e h t c j 3 1w / c ) 8 2 - p o s s t ( t n e i b m a o t n o i t c n u j e c n a t s i s e r l a m r e h t a j 4 8w / c e g n a r e r u t a r e p m e t e g a r o t st g t s 0 5 1 o t 0 6 -c c e s 0 1 ) g n i r e d l o s ( e r u t a r e p m e t d a e lt d a e l 0 6 2c e r u t a r e p m e t n o i t c n u j m u m i x a m t j 0 5 1c unless specified: avcc = 12v, sync = 0, r osc = 51.1k ? , -40c < t a = t j < 125c 3 ? 2005 semtech corp. www.semtech.com power management SC2446A electrical characteristics (cont.) r e t e m a r a pl o b m y ss n o i t i d n o cn i mp y tx a ms t i n u t n e r r u c k n i s t u p t u o r e i f i l p m av - 1 n i v , v 1 = 1 p m o c v 5 . 2 =6 1 a t n e r r u c e c r u o s t u p t u o r e i f i l p m av - 1 n i v , 0 = 1 p m o c v 5 . 2 =2 1 a r e i f i l p m a r o r r e 2 l e n n a h c e g n a r e g a t l o v e d o m - n o m m o c t u p n i) 1 e t o n (03v e g n a r e g a t l o v t u p n i g n i t r e v n i) 1 e t o n (0c c v av e g a t l o v t e s f f o t u p n ic 0 7 ~ 05 . 13 v m t n e r r u c s a i b t u p n i g n i t r e v n i - n o ni + 2 n i 0 5 1 -0 8 3 -a n t n e r r u c s a i b t u p n i g n i t r e v n ii - 2 n i 0 0 1 -0 5 2 -a n e l g n i s e s a h p - 2 r o f e g a t l o v t u p n i g n i t r e v n i n o i t a r e p o t u p t u o 5 . 2v e c n a t c u d n o c s n a r t r e i f i l p m ag 2 m 0 6 2 ? 1 ? n i a g p o o l - n e p o r e i f i l p m aa 2 l o 5 6d h t d i w d n a b n i a g y t i n u r e i f i l p m a 5z h m d l o h s e r h t g n i h c t i w s 2 p m o c m u m i n i m v + 2 s c v = - 2 s c 0 = v 2 s s g n i s a e r c n i 2 . 2v t n e r r u c k n i s t u p t u o r e i f i l p m av 2 p m o c v 5 . 2 =6 1 a t n e r r u c e c r u o s t u p t u o r e i f i l p m av 2 p m o c v 5 . 2 =2 1 a r o t a l l i c s o y c n e u q e r f l e n n a h cf , 1 h c f 2 h c c 0 7 ~ 00 5 40 0 50 5 5z h k y c n e u q e r f g n i z i n o r h c n y s) 1 e t o n (f 1 . 2 h c z h k e g a t l o v h g i h t u p n i c n y s 5 . 1v e g a t l o v w o l t u p n i c n y s 5 . 0v t n e r r u c t u p n i c n y si c n y s v c n y s v 2 . 0 = v c n y s v 2 = 1 0 5 a e l c y c y t u d m u m i x a m l e n n a h cd , 1 x a m d 2 x a m 8 8% e l c y c y t u d m u m i n i m l e n n a h cd , 1 n i m d 2 n i m 0% s r o t a r a p m o c t i m i l - t n e r r u c e g n a r e d o m - n o m m o c t u p n i 01 - c c v av t i m i l t n e r r u c k a e p e l c y c - y b - e l c y c v , + 1 m i l i v + 2 m i l i v - 1 s c v = - 2 s c , v 5 . 0 = c 0 7 ~ 0 , e d o m g n i c r u o s 0 40 50 6v m n w o d t u h s d a o l r e v o t n e r r u c y e l l a v d l o h s e r h t v , - 1 m i l i v - 2 m i l i v - 1 s c v = - 2 s c , v 5 . 0 = c 0 7 ~ 0 , e d o m g n i k n i s 0 6 -5 7 -0 9 -v m t n e r r u c s a i b t u p n i e s n e s - t n e r r u c e v i t i s o pi , + 1 s c i + 2 s c v + 1 s c =v - 1 s c 0 = v - 2 s c =v - 2 s c 0 = 7 . 0 -2 - a s a i b t u p n i e s n e s - t n e r r u c e v i t a g e n t n e r r u c i , - 1 s c i - 2 s c v + 1 s c =v - 1 s c 0 = v + 2 s c =v - 2 s c 0 = 7 . 0 -2 - a unless specified: avcc = 12v, sync = 0, r osc = 51.1k ? , -40c < t a = t j < 125c 4 ? 2005 semtech corp. www.semtech.com power management SC2446A notes: (1) guaranteed by design not tested in production. (2) this device is esd sensitive. use of standard esd handling precautions is required. electrical characteristics (cont.) r e t e m a r a pl o b m y ss n o i t i d n o cn i mp y tx a ms t i n u s t u p t u o m w p t n e r r u c e c r u o s k a e pg 1 l d g , , 2 l d g 1 h d g , 2 h d v 2 1 = c c v a4a m t n e r r u c k n i s k a e pg 1 l d g , , 2 l d g 1 h d g , 2 h d v 2 1 = c c v a3a m e g a t l o v h g i h t u p t u oi e c r u o s o c 0 7 ~ 0 , a m 2 . 1 =5 9 . 35v e g a t l o v w o l t u p t u o i k n i s o a m 1 =0 4 . 0 v e m i t - n o m u m i n i mt a c 5 2 =0 2 1s n e l b a n e d n a f f o h c t a l d a o l r e v o , t r a t s - t f o s t n e r r u c g n i g r a h c t r a t s - t f o si , 1 s s i 2 s s v 1 s s =v 2 s s =v 5 . 18 . 1 a g n i l b a n e f f o h c t a l d a o l r e v o e g a t l o v t r a t s - t f o s v 1 s s d n av 2 s s g n i s a e r c n i2 . 3v f f o h c t a l d a o l r e v o d l o h s e r h t - 1 n i v 1 s s v , v 8 . 3 = - 1 n i g n i s a e r c e dv 5 . 0 f e r v f f o h c t a l d a o l r e v o d l o h s e r h t - 2 n i v 2 s s v , v 8 . 3 = - 2 n i g n i s a e r c e d x 5 . 0 v n i f e r v t n e r r u c e g r a h c s i d t r a t s - t f o si , ) s i d ( 1 s s i ) s i d ( 2 s s v - 1 n i =v 5 . 0 f e r , v - 2 n i =v 5 . 0 n i f e r , v 1 s s =v 2 s s =v 8 . 3 2 . 1 a y r e v o c e r f f o h c t a l d a o l r e v o e g a t l o v t r a t s - t f o s v , 1 v c r s s v 2 v c r s s v 1 s s d n av 2 s s g n i s a e r c e d3 . 05 . 07 . 0v n e / s s e l b a s i d t u p t u o m w p e g a t l o v 7 . 08 . 0v n e / s s e l b a n e t u p t u o m w p e g a t l o v 2 . 15 . 1v r e f f u b e c n e r e f e r v 5 . 0 l a n r e t n i e g a t l o v t u p t u ov t u o f e r i t u o f e r t < c 0 , a m 1 - = a t = j c 0 7 <5 9 40 0 55 0 5v m n o i t a l u g e r d a o l i < 0 t u o f e r a m 5 - <5 0 . 0a m / % n o i t a l u g e r e n i l c c v a h t i , v 5 1 < c c v a < t u o f e r a m 1 - =% 2 0 . 0v % unless specified: avcc = 12v, sync = 0, r osc = 51.1k ? , -40c < t a = t j < 125c - 5 ? 2005 semtech corp. www.semtech.com power management SC2446A pin configurations ordering information e c i v e de g a k c a pt ( e g n a r . p m e t a ) t r t s t i a 6 4 4 2 c s ) 2 ( ) 1 ( 8 2 - p o s s tc 5 2 1 o t 0 4 - b v e a 6 4 4 2 c sd r a o b n o i t a u l a v e notes: (1) only available in tape and reel packaging. a reel contains 2500 devices for tssop package. (2) lead free product. this product is fully weee and rohs compliant. figure 2 1 2 3 4 5 6 7 8 ss1/en1 cs1+ top view (28 pin tssop) 27 28 15 16 cs1- rosc gdh1 in1- comp1 pvcc sync pgnd agnd ref 9 10 22 gdh2 refout refin 21 18 17 19 20 11 12 24 comp2 in2- 23 25 26 13 14 avcc cs2- ss2/en2 cs2+ gdl1 bst1 vpn1 vpn2 bst2 gdl2 vin2 6 ? 2005 semtech corp. www.semtech.com power management SC2446A pin descriptions n i pe m a n n i pn o i t c n u f n i p 1+ 1 s c . 1 r e l l o r t n o c e h t r o f r o t a r a p m o c / r e i f i l p m a e s n e s - t n e r r u c e h t f o t u p n i g n i t r e v n i - n o n e h t 2- 1 s c y l l a m r o n . 1 r e l l o r t n o c e h t r o f r o t a r a p m o c / r e i f i l p m a e s n e s - t n e r r u c e h t f o t u p n i g n i t r e v n i e h t . r e t r e v n o c e h t f o t u p t u o e h t o t d e i t 3c s o r . y c n e u q e r f r o t a l l i c s o e h t s t e s d n g o t n i p s i h t m o r f d e t c e n n o c r o t s i s e r l a n r e t x e n a 4- 1 n i e v i t s i s e r l a n r e t x e n a e i t . 1 r e l l o r t n o c n w o d - p e t s e h t r o f r e i f i l p m a r o r r e e h t f o t u p n i g n i t r e v n i . g n i s n e s e g a t l o v t u p t u o r o f d n u o r g e h t d n a 1 t u p t u o n e e w t e b r e d i v i d 51 p m o c . n o i t a s n e p m o c p o o l r o f d e s u s i n i p s i h t . 1 r e l l o r t n o c n w o d - p e t s r o f t u p t u o r e i f i l p m a r o r r e e h t 6c n y s e v o b a e g a t l o v a o t n i p s i h t e i t , d e z i n o r h c n y s t o n n e h w . t u p n i n o i t a z i n o r h c n y s d e r e g g i r t - e g d e n i p s i h t t a ) c s o r h t i w t e s y c n e u q e r f > y c n e u q e r f ( k c o l c l a n r e t x e n a . d n u o r g e h t r o v 5 . 1 . s r e l l o r t n o c e h t s e z i n o r h c n y s 7d n g a. d n u o r g l a n g i s g o l a n a 8f e r . 1 r e t r e v n o c n w o d p e t s e h t r o f r e i f i l p m a r o r r e e h t f o t u p n i g n i t r e v n i - n o n e h t 9t u o f e r. v 5 . 0 e g a t l o v e c n e r e f e r l a n r e t n i e h t f o t u p t u o d e r e f f u b 0 1n i f e r r o r r e e h t f o t u p n i g n i t r e v n i - n o n e h t . n i p s i h t o t d e i l p p a s i e g a t l o v e c n e r e f e r l a n r e t x e n a . n i p s i h t o t d e t c e n n o c y l l a n r e t n i s i 2 r e t r e v n o c n w o d - p e t s e h t r o f r e i f i l p m a 1 12 p m o c . n o i t a s n e p m o c p o o l r o f d e s u s i n i p s i h t . 2 r e l l o r t n o c n w o d - p e t s r o f t u p t u o r e i f i l p m a r o r r e e h t 2 1- 2 n i e v i t s i s e r l a n r e t x e n a e i t . 2 r e l l o r t n o c n w o d - p e t s e h t r o f r e i f i l p m a r o r r e e h t f o t u p n i g n i t r e v n i e s a h p - o w t r o f c c v a o t e i t . g n i s n e s e g a t l o v t u p t u o r o f d n u o r g e h t d n a 2 t u p t u o n e e w t e b r e d i v i d s n o i t a c i l p p a t u p t u o e l g n i s 3 1- 2 s c y l l a m r o n . 2 r e l l o r t n o c e h t r o f r o t a r a p m o c / r e i f i l p m a e s n e s - t n e r r u c e h t f o t u p n i g n i t r e v n i e h t . r e t r e v n o c e h t f o t u p t u o e h t o t d e i t 4 1+ 2 s c 2 r e l l o r t n o c e h t r o f r o t a r a p m o c / r e i f i l p m a e s n e s - t n e r r u c e h t f o t u p n i g n i t r e v n i - n o n e h t 5 12 n e / 2 s s e m i t f f o h c t a l d a o l r e v o t u p t u o ) i i ( e m i t t r a t s - t f o s e h t ) i ( s t e s n i p s i h t o t d e i t r o t i c a p a c l a n r e t x e n a d n o c e s e h t r o f s r e v i r d e t a g e h t f f o s t u h s v 7 . 0 w o l e b n i p s i h t g n i l l u p . 2 r e t r e v n o c n w o d - p e t s r o f . s n o i t a c i l p p a t u p t u o e l g n i s e s a h p - o w t r o f n e p o e v a e l . r e l l o r t n o c 6 1c c v a . s r e l l o r t n o c e h t f o n o i t r o p g o l a n a e h t r o f e g a t l o v y l p p u s r e w o p 7 12 n i v . n o i t c e n n o c o n 8 12 n p v . n o i t c e n n o c o n 9 12 t s b . n o i t c e n n o c o n 0 22 h d g . 2 t u p t u o f o t e f s o m l e n n a h c - n e d i s - h g i h e h t r o f t u p t u o m w p tssop package 7 ? 2005 semtech corp. www.semtech.com power management SC2446A pin descriptions n i pe m a n n i pn o i t c n u f n i p 1 22 l d g . 2 t u p t u o r o f l a n g i s e v i r d e t a g e l b a n e c i g o l 2 2d n g p. n o i t c e n n o c o n 3 2c c v p. n o i t c e n n o c o n 4 21 l d g. 1 t u p t u o r o f l a n g i s e v i r d e t a g e l b a n e c i g o l 5 21 h d g. 1 t u p t u o f o t e f s o m l e n n a h c - n e d i s - h g i h e h t r o f t u p t u o m w p 6 21 t s b. n o i t c e n n o c o n 7 21 n p v. n o i t c e n n o c o n 8 21 n e / 1 s s e m i t f f o h c t a l d a o l r e v o t u p t u o ) i i ( e m i t t r a t s - t f o s e h t ) i ( s t e s n i p s i h t o t d e i t r o t i c a p a c l a n r e t x e n a . r e l l o r t n o c t s r i f e h t r o f s r e v i r d e t a g e h t f f o s t u h s v 7 . 0 w o l e b n i p s i h t g n i l l u p . 1 r e t r e v n o c k c u b r o f 8 ? 2005 semtech corp. www.semtech.com power management SC2446A block diagram figure 3. SC2446A block diagram vin2 bst1 25 26 27 22 28 16 24 17 20 19 18 21 sync oscillator ea1 slope1 + clk2 r q s pwm1 pvcc 23 rosc in1- cs1+ cs1- clk1 - + - + + ref + - isen1 avcc gdh1 gdl1 pgnd ss1/en1 comp1 vpn1 2 4 3 6 1 uv agnd 7 5 frequency divider clk slope comp slope2 gdh2 gdl2 ss2/en2 vpn2 15 bst2 ea2 slope2 i + ilim2 r q s pwm2 in2- cs2+ cs2- - + - + + + - isen2 + - 50mv comp2 13 12 14 uv 11 sel clk2 1.8v - + analog switch sel a y b 8 refout reference 0.5v 1.25v 0.5v 9 50mv 75mv i ilim1+ + - i ilim1+ + - ilim1 - ocn1 0.5 (refout) 0.5v + - refin 10 ol1 soft-start and overload hiccup control 1 dsbl1 75mv ilim2 - ocn2 + - 0.5 (refin) ol2 dsbl2 soft-start and overload hiccup control 2 0.5 (refin) uvlo 4.3/4.5v + - + - 9 ? 2005 semtech corp. www.semtech.com power management SC2446A figure 4. soft-start and overload hiccup control circuit ol 0.5v/3.2v s q r - 0.8v/1.2v dsbl 1.8? 3? in- ocn ss/en uvlo / 0.5(v refin ) + 0.5(v refout ) block diagram 10 ? 2005 semtech corp. www.semtech.com power management SC2446A SC2446A consists of two current-mode synchronous buck controllers with many integrated functions. by proper application circuitry configuration, SC2446A can be used to generate 1) two independent outputs from a common input or two different inputs or 2) dual phase output with current sharing, 3) current sourcing/sinking from common or separate inputs as in ddr (i and ii) memory application. the application information related to the converter design using SC2446A is described in the following. step-down converter starting from the following step-down converter specifications, input voltage range: ] v , v [ v max , in min , in in input voltage ripple (peak-to-peak): ? v in output voltage: v o output voltage accuracy: output voltage ripple (peak-to-peak): ? v o nominal output (load) current: i o maximum output current limit: i o,max output (load) current transient slew rate: di o (a/s) circuit efficiency: selection criteria and design procedures for the following are described. 1) output inductor ( l ) type and value, 2) output capacitor ( c o ) type and value, 3) input capacitor ( c in ) type and value, 4) power mosfet?s, 5) current sensing and limiting circuit, 6) voltage sensing circuit, 7) loop compensation network. operating frequency (f s ) the switching frequency in the SC2446A is user- programmable. the advantages of using constant frequency operation are simple passive component selection and ease of feedback compensation. before setting the operating frequency, the following trade-offs should be considered. 1) passive component size 2) circuitry efficiency 3) emi condition 4) minimum switch on time and 5) maximum duty ratio for a given output power, the sizes of the passive components are inversely proportional to the switching frequency, whereas mosfets/diodes switching losses are proportional to the operating frequency. other issues such as heat dissipation, packaging and the cost issues are also to be considered. the frequency bands for signal transmission should be avoided because of em interference. minimum switch on time consideration in the SC2446A the falling edge of the clock turns on the top mosfet gate. the inductor current and the sensed voltage ramp up. after the sensed voltage crosses a threshold determined by the error amplifier output, the top mosfet gate is turned off. the propagation delay time from the turn-on of the controlling fet to its turn- off is the minimum switch on time. the SC2446A has a minimum on time of about 120ns at room temperature. this is the shortest on interval of the controlling fet. the controller either does not turn on the top mosfet at all or turns it on for at least 120ns. for a synchronous step-down converter, the operating duty cycle is v o /v in . so the required on time for the top mosfet is v o /(v in fs). if the frequency is set such that the required pulse width is less than 120ns, then the converter will start skipping cycles. due to minimum on time limitation, simultaneously operating at very high switching frequency and very short duty cycle is not practical. if the voltage conversion ratio v o /v in and hence the required duty cycle is higher, the switching frequency can be increased to reduce the sizes of passive components. there will not be enough modulation headroom if the on time is simply made equal to the minimum on time of the SC2446A. for ease of control, we recommend the required pulse width to be at least 1.5 times the minimum on time. application information 11 ? 2005 semtech corp. www.semtech.com power management SC2446A setting the switching frequency the switching frequency is set with an external resistor connected from pin 3 to the ground. the set frequency is inversely proportional to the resistor value (figure 5). 0 100 200 300 400 500 600 700 800 0 50 100 150 200 250 rosc (k ohm) fs (khz) figure 5. free running frequency vs. r osc . inductor (l) and ripple current both step-down controllers in the SC2446A operate in synchronous continuous-conduction mode (ccm) regardless of the output load. the output inductor selection/design is based on the output dc and transient requirements. both output current and voltage ripples are reduced with larger inductors but it takes longer to change the inductor current during load transients. conversely smaller inductors results in lower dc copper losses but the ac core losses (flux swing) and the winding ac resistance losses are higher. a compromise is to choose the inductance such that peak-to-peak inductor ripple-current is 20% to 30% of the rated output load current. assuming that the inductor current ripple (peak-to-peak) value is * i o , the inductance value will then be . f i ) d 1 ( v l s o o ? = the peak current in the inductor becomes (1+ /2)*io and the rms current is . 12 1 i i 2 o rms , l + = the followings are to be considered when choosing inductors. a) inductor core material: for high efficiency applications above 350khz, ferrite, kool-mu and polypermalloy materials should be used. low-cost powdered iron cores can be used for cost sensitive-applications below 350khz but with attendant higher core losses. b) select inductance value: sometimes the calculated inductance value is not available off-the-shelf. the designer can choose the adjacent (larger) standard inductance value. the inductance varies with temperature and dc current. it is a good engineering practice to re-evaluate the resultant current ripple at the rated dc output current. c) current rating: the saturation current of the inductor should be at least 1.5 times of the peak inductor current under all conditions. output capacitor (c o ) and v out ripple the output capacitor provides output current filtering in steady state and serves as a reservoir during load transient. the output capacitor can be modeled as an ideal capacitor in series with its parasitic esr ( r esr ) and esl ( l esl ) (figure 6). co lesl resr figure 6. an equivalent circuit of c o . if the current through the branch is i b (t), the voltage across the terminals will then be ). t ( i r dt ) t ( di l dt ) t ( i c 1 v ) t ( v b esr t 0 b esl b o o o + + + = this basic equation illustrates the effect of esr, esl and c o on the output voltage. the first term is the dc voltage across c o at time t=0 . the second term is the voltage variation caused by the application information (cont.) 12 ? 2005 semtech corp. www.semtech.com power management SC2446A charge balance between the load and the converter output. the third term is voltage ripple due to esl and the fourth term is the voltage ripple due to esr. the total output voltage ripple is then a vector sum of the last three terms. since the inductor current is a triangular waveform with peak-to-peak value * i o , the ripple-voltage caused by inductor current ripples is , f c 8 i v s o o c ? the ripple-voltage due to esl is , d i f l v o s esl esl = ? and the esr ripple-voltage is . i r v o esr esr = ? aluminum capacitors (e.g. electrolytic, solid os-con, poscap, tantalum) have high capacitances and low esl?s. the esr has the dominant effect on the output ripple voltage. it is therefore very important to minimize the esr. when determining the esr value, both the steady state ripple-voltage and the dynamic load transient need to be considered. to keep the steady state output ripple-voltage < ? v o , the esr should satisfy . i v r o o 1 esr ? < to limit the dynamic output voltage overshoot/ undershoot within (say 3%) of the steady state output voltage) from no load to full load, the esr value should satisfy . i v r o o 2 esr < then, the required esr value of the output capacitors should be r esr = min{r esr1 ,r esr2 }. the voltage rating of aluminum capacitors should be at least 1.5 v o . the rms current ripple rating should also be greater than . 3 2 i o usually it is necessary to have several capacitors of the same type in parallel to satisfy the esr requirement. the voltage ripple cause by the capacitor charge/discharge should be an order of magnitude smaller than the voltage ripple caused by the esr. to guarantee this, the capacitance should satisfy . r f 2 10 c esr s o > in many applications, several low esr ceramic capacitors are added in parallel with the aluminum capacitors in order to further reduce esr and improve high frequency decoupling. because the values of capacitance and esr are usually different in ceramic and aluminum capacitors, the following remarks are made to clarify some practical issues. remark 1: high frequency ceramic capacitors may not carry most of the ripple current. it also depends on the capacitor value. only when the capacitor value is set properly, the effect of ceramic capacitor low esr starts to be significant. for example, if a 10 f, 4m ? ceramic capacitor is connected in parallel with 2x1500 f, 90m ? electrolytic capacitors, the ripple current in the ceramic capacitor is only about 42% of the current in the electrolytic capacitors at the ripple frequency. if a 100 f, 2m ? ceramic capacitor is used, the ripple current in the ceramic capacitor will be about 4.2 times of that in the electrolytic capacitors. when two 100 f, 2m ? ceramic capacitors are used, the current ratio increases to 8.3. in this case most of the ripple current flows in the ceramic decoupling capacitor. the esr of the ceramic capacitors will then determine the output ripple-voltage. remark 2: the total equivalent capacitance of the filter bank is not simply the sum of all the paralleled capacitors. the total equivalent esr is not simply the parallel combination of all the individual esr?s either. instead they should be calculated using the following formulae. application information (cont.) 13 ? 2005 semtech corp. www.semtech.com power management SC2446A ) c c ( c c ) c r c r ( ) c c ( c c ) r r ( : ) ( c b 1 a 1 b 1 a 1 2 b 1 2 b 1 a 1 2 a 1 2 b 1 a 1 2 b 1 2 a 1 2 2 b 1 a 1 eq + + + + + + = 2 b 1 a 1 2 b 1 2 a 1 2 2 b 1 a 1 2 a 1 a 1 2 b 1 b 1 2 b 1 2 a 1 2 b 1 a 1 b 1 a 1 eq ) c c ( c c ) r r ( ) c r c r ( c c ) r r ( r r : ) ( r + + + + + + = where r 1a and c 1a are the esr and capacitance of electrolytic capacitors, and r 1b and c 1b are the esr and capacitance of the ceramic capacitors respectively. (figure 7) c1a r1a c1b r1b ceq req figure 7. equivalent rc branch. req and ceq are both functions of frequency. for rigorous design, the equivalent esr should be evaluated at the ripple frequency for voltage ripple calculation when both ceramic and electrolytic capacitors are used. if r 1a = r 1b = r 1 and c 1a = c 1b = c 1 , then r eq and c eq will be frequency- independent and r eq = 1/2 r 1 and c eq = 2c 1 . input capacitor (c in ) the input supply to the converter usually comes from a pre-regulator. since the input supply is not ideal, input capacitors are needed to filter the current pulses at the switching frequency. a simple buck converter is shown in figure 8. figure 8. a simple model for the converter input in figure 8 the dc input voltage source has an internal impedance r in and the input capacitor c in has an esr of r esr . mosfet and input capacitor current waveforms, esr voltage ripple and input voltage ripple are shown in figure 9. figure 9. typical waveforms at converter input. it can be seen that the current in the input capacitor pulses with high di/dt. capacitors with low esl should be used. it is also important to place the input capacitor close to the mosfets on the pc board to reduce trace inductances around the pulse current loop. the rms value of the capacitor current is approximately ]. ) d 1 ( d ) d 1 )( 12 1 [( d i i 2 2 2 o cin ? + ? + = application information (cont.) 14 ? 2005 semtech corp. www.semtech.com power management SC2446A the power dissipated in the input capacitors is then p cin = i cin 2 r esr . for reliable operation , the maximum power dissipation in the capacitors should not result in more than 10 o c of temperature rise. many manufacturers specify the maximum allowable ripple current (arms) rating of the capacitor at a given ripple frequency and ambient temperature. the input capacitance should be high enough to handle the ripple current. for higher power applications, multiple capacitors are placed in parallel to increase the ripple current handling capability. sometimes meeting tight input voltage ripple specifications may require the use of larger input capacitance. at full load, the peak-to-peak input voltage ripple due to the esr is . i ) 2 1 ( r v o esr esr + = ? the peak-to-peak input voltage ripple due to the capacitor is , f c di v s in o c ? from these two expressions, c in can be found to meet the input voltage ripple specification. in a multi-phase converter, channel interleaving can be used to reduce ripple. the two step-down channels of the SC2446A operate at 180 degrees from each other. if both step- down channels in the SC2446A are connected in parallel, both the input and the output rms currents will be reduced. ripple cancellation effect of interleaving allows the use of smaller input capacitors. when converter outputs are connected in parallel and interleaved, smaller inductors and capacitors can be used for each channel. the total output ripple-voltage remains unchanged. smaller inductors speeds up output load transient. when two channels with a common input are interleaved, the total dc input current is simply the sum of the individual dc input currents. the combined input current waveform depends on duty ratio and the output current waveform. assuming that the output current ripple is small, the following formula can be used to estimate the rms value of the ripple current in the input capacitor. let the duty ratio and output current of channel 1 and channel 2 be d 1 , d 2 and i o1 , i o2 , respectively. if d 1 <0.5 and d 2 <0.5, then . i d i d i 2 2 o 2 2 1 o 1 cin + choosing power mosfets the power devices with integrated gate drivers such as pip212, r2j20601np, pip202, pip201 and ip2001, ip2002 are suitable for SC2446A application. current sensing inductor current sensing is required for the current-mode control. although the inductor current can be sensed with a precision resistor in series with the inductor, the lossless inductive current sense technique is used in the SC2446A. this technique has the advantages of 1) lossless current sensing, 2) lower cost compared to resistive sense 3) more accurate compared to the r ds(on) sense the basic arrangement of the inductive current sense is shown in figure 10. where, r l is the equivalent series resistance of the output inductor. the added r s and c s form a rc branch for inductor current sensing. q1 q2 cin cout rl rload rs vin vo cs vc(t) l il(t) figure 10. the basic structure of inductive current sense. in steady state, the dc value, v cs = r l i o application information (cont.) 15 ? 2005 semtech corp. www.semtech.com power management SC2446A it is noted that the dc value of v cs is independent of the value of l, r s and c s . this means that, if only the average load current information is needed (such as in average current mode control), this current sensing method is effective without time constant matching requirement. in the current mode control as implemented in SC2446A, the voltage ripple on c s is critical for pwm operation. in fact, the ac voltage ripple peak-to-peak value of v cs (denoted as ? v cs ) directly effects the signal-to-noise ratio of the pwm operation. in general, smaller ? v cs leads to lower signal-to-noise ratio and more noise sensitive operation. larger ? v cs leads to more circuit (power stage) parameter sensitive operation. a good engineering compromise is to make ? v cs ~ r l i o . the pre-requisite for such relation is the so called time constant matching condition . c r r l s s l for an example of application circuit, l = 1h, rl = 1.8m ?, the time constant r s c s should be set as 555.6s. if one selects c s = 33nf, then r s = 16.9 k ? . scaling the current limit over-current is handled differently in the SC2446A depending on the direction of the inductor current. if the differential sense voltage between cs+ and cs- exceeds +50mv, the top mosfet will be turned off and the bottom mosfet will be turned on to limit the inductor current. this +50mv is the cycle-by-cycle peak current limit when the load is drawing current from the converter. there is no cycle-by-cycle current limit when the inductor current flows in the reverse direction. if the voltage between cs1+ and cs- falls below -75mv, the controller will undergo overload shutdown and time-out with both the top and the bottom mosfets shut off. (see the section overload protection and hiccup). in the circuit of figure 11, the equivalent inductor current limits are set according to , r mv 50 i l lmcp = when the load is sourcing current from the converter and , r mv 75 i l lmcn ? = when the load is forcing current back to the input power source. if r l = 1.8m ? , then i lm = 27.8 / -41.7a. the circuit in figure 11 allows the user to scale the equivalent current limit with the same rl. in the following design steps, the capacitor c s in the current sensing part is commonly selected in the range of 22nf ~ 100nf. q1 q2 cin cout rl rload rs vin vo cs vc(t) l il(t) vgs2 vgs1 pn 1 2 - + isen rs3 rs2 rs1 figure 11. scaling the equivalent current limit. a) when the required current limit value i lm is greater than i lmcp , one just needs to remove r s3 , and solve the following equations , r l c ) r // r ( l s 1 s s = , mv 50 r r r r i 1 s s 1 s l lm = + and . r // r r 1 s s 2 s = for . r and r , r 2 s 1 s s application information (cont.) 16 ? 2005 semtech corp. www.semtech.com power management SC2446A note that r s2 is selected as r s //r s1 in order to reduce the bias current effect of the current amplifier in SC2446A. b) when the required current limit i lm is less than i lmcp , one just needs to remove r s1 and solve , r l c r l s s = , mv 50 v r r r i o 3 s s l lm = + for r s and r s3 . r s2 is then obtained from . r r r r r s 3 s s 3 s 2 s ? = similar steps and equations apply to the current limit setting and scaling for current sinking mode. overload protection and hiccup during start-up, the capacitor from the ss/en pin to ground functions as a soft-start capacitor. after the converter starts and enters regulation, the same capacitor operates as an overload shutoff timing capacitor. as the load current increases, the cycle-by- cycle current-limit comparator will first limit the inductor current. further increase in loading will cause the output voltage (hence the feedback voltage) to fall. if the feedback voltage falls to less than (50% for ch1, 50% for ch2) of the reference voltage, the controller will shut off both the top and the bottom mosfets. meanwhile an internal net 1.2 a current source discharges the soft- start capacitor c 32 (c 33 ) connected to the ss/en pin. when the capacitor is discharged to 0.5v, a 1.8 a current source recharges the ss/en capacitor and converter restarts. if overload persists, the controller will shut down the converter when the soft start capacitor voltage exceeds 3.2v. the converter will repeatedly start and shut off until it is no longer overloaded. this hiccup mode of overload protection is a form of foldback current limiting. the following calculations estimate the average inductor current when the converter output is shorted to the ground. a) the time taken to discharge the capacitor from 3.2v to 0.5v . a 2 . 1 v ) 5 . 0 2 . 3 ( c t 32 ssf ? = if c 32 = 0.1 f, t ssf is calculated as 225ms. b) the soft start time from 0.5v to 3.2v . a 8 . 1 v ) 5 . 0 2 . 3 ( c t 32 ssr ? = when c 32 = 0.1 f, t ssr is calculated as 150ms. note that during soft start, the converter only starts switching when the voltage at ss/en exceeds 1.2v. c) the effective start-up time is . a 8 . 1 v ) 2 . 1 2 . 3 ( c t 32 sso ? = the average inductor current is then . t t t i i ssr ssf sso lmcp leff + = i leff 0.30 i lmcp and is independent of the soft start capacitor value. the converter will not overheat in hiccup. setting the output voltage the non-inverting input of the channel-one error amplifier is internally tied the 0.5v voltage reference output (pin 8). the non-inverting input of the channel-two error amplifier is brought out as a device pin (pin 10) to which the user can connect pin 8 or an external voltage reference. a simple voltage divider (r o1 at top and r o2 at bottom) sets the converter output voltage. the voltage feedback gain h=0.5/v o is related to the divider resistors value as . 1 o 2 o r h 1 h r ? = once either r o1 or r o2 is chosen, the other can be calculated for the desired output voltage v o . since the number of standard resistance values is limited, the calculated resistance may not be available as a standard value resistor. as a result, there will be a set error in the application information (cont.) 17 ? 2005 semtech corp. www.semtech.com power management SC2446A converter output voltage. this non-random error is caused by the feedback voltage divider ratio. it cannot be corrected by the feedback loop. the following table lists a few standard resistor combinations for realizing some commonly used output voltages. ) v ( o v 6 . 0 9 . 0 2 . 15 . 1 8 . 15 . 23 . 3 h / ) h - 1 ( 2 . 0 8 . 0 4 . 12 6 . 24 6 . 5 ) m h o ( 1 o r 0 0 2 6 0 8 k 4 . 1k 2 k 1 6 . 2k 2 0 . 4k 2 6 . 5 ) m h o ( 2 o r k 1 k 1 k 1k 1 k 1k 1k 1 only the voltages in boldface can be precisely set with standard 1% resistors. from this table, one may also observe that when the value 5 . 0 5 . 0 v h h 1 o ? = ? and its multiples fall into the standard resistor value chart (1%, 5% or so), it is possible to use standard value resistors to exactly set up the required output voltage value. the input bias current of the error amplifier also causes an error in setting the output voltage. the maximum inverting input bias currents of error amplifiers 1 and 2 is -250na. since the non-inverting input is biased to 0.5v, the percentage error in the second output voltage will be ?100% (0.25 a) r o1 r o2 /[0.5 (r o1 +r o2 ) ]. to keep this error below 0.2%, r o2 < 4k ? . loop compensation SC2446A uses current-mode control for both step-down channels. current-mode control is a dual-loop control system in which the inductor peak current is loosely controlled by the inner current-loop. the higher gain outer loop regulates the output voltage. since the current loop makes the inductor appear as a current source, the complex high-q poles of the output lc networks is split into a dominant pole determined by the output capacitor and the load resistance and a high frequency pole. this pole-splitting property of current-mode control greatly simplifies loop compensation. the inner current-loop is unstable (sub-harmonic oscillation) unless the inductor current up-slope is steeper than the inductor current down-slope. for stable operation above 50% duty-cycle, a compensation ramp is added to the sensed-current. in the SC2446A the compensation ramp is made duty-ratio dependent. the compensation ramp is approximately . a 30 * de i d 76 . 1 ramp = the slope of the compensation ramp is then . a 30 * f e ) d 76 . 1 1 ( s s d 76 . 1 e + = the slope of the internal compensation ramp is well above the minimal slope requirement for current loop stability and is sufficient for all the applications. with the inner current loop stable, the output voltage is then regulated with the outer voltage feedback loop. a simplified equivalent circuit model of the synchronous buck converter with current mode control is shown in figure 12. k figure 12. a simple model of synchronous buck converter with current mode control. the transconductance error amplifier (in the SC2446A) has a gain g m of 260 a/v. the target of the compensation design is to select the compensation network consisting of c 2 , c 3 and r 2 , along with the feedback resistors r o1 , application information (cont.) 18 ? 2005 semtech corp. www.semtech.com power management SC2446A r o2 and the current sensing gain, such that the converter output voltage is regulated with satisfactory dynamic performance. with the output voltage v o known, the feedback gain h and the feedback resistor values are determined using the equations given in the ?output voltage setting? section with . v 5 . 0 h o = for the rated output current i o , the current sensing gain k is first estimated as . 1 . 2 i k o = from the transfer function from the voltage error amplifier output v c to the converter output v o is . s s 1 s s 1 kr ) s ( g : ) s ( v ) s ( v 1 p 1 z o vc c o + + = = where, the single dominant pole is , c ) r r ( 1 s o oesr o 1 p + = and the zero due to the output capacitor esr is . c r 1 s o oesr 1 z = the dominant pole moves as output load varies. the controller transfer function (from the converter output v o to the voltage error amplifier output v c ) is , s s 1 s s 1 ) c c ( s h g ) s ( c 2 p 2 z 3 2 m + + + = where , c r 1 s 2 2 2 z = and . c c c c r 1 s 3 2 3 2 2 2 p + = the loop transfer function is then t(s)=g vc (s)c(s). to simplify design, we assume that c 3 < 20 ? 2005 semtech corp. www.semtech.com power management SC2446A pc board layout issues circuit board layout is very important for the proper operation of high frequency switching power converters. a power ground plane is required to reduce ground bounces. the followings are suggested for proper layout. power stage 1) separate the power ground from the signal ground. in SC2446A, the power ground pgnd should be tied to the source terminal of lower mosfets. the signal ground agnd should be tied to the negative terminal of the output capacitor. 2) minimize the size of high pulse current loop. keep the top mosfet, bottom mosfet and the input capacitors within a small area with short and wide traces. in addition to the aluminum energy storage capacitors, add multi- layer ceramic (mlc) capacitors from the input to the power ground to improve high frequency bypass. control section 3) the frequency-setting resistor rosc should be placed close to pin 3. trace length from this resistor to the analog ground should be minimized. 4) solder the bias decoupling capacitor right across the avcc and analog ground agnd. 5) place the combi-sense components away from the power circuit and close to the corresponding cs+ and cs- pins. use x7r type ceramic capacitor for the combi- sense capacitor because of their temperature stability. 6) use an isolated local ground plane for the controller and tie it to the negative side of output capacitor bank. application information (cont.) 21 ? 2005 semtech corp. www.semtech.com power management SC2446A typical application schematic l2 1uh vo1gnd c26 100uf r16 2.49k r19 12.4k c31 6.8nf c28 33pf vin (12v) vo2gnd c29 0.1uf r20 49.9k vo2 (1.2v, 15a) c20 100nf c46 100uf rcs-2 20k c38 100nf r5 20k rcs-1 20k c21 100nf r6 20k r26 10 c47 100uf r9 2.2m r11 10k c36 33pf r21 12.4k c37 6.8nf r24 10 rcs+1 49.9k c45 0.1uf c39 100nf r17 7.15k c9 10uf c10 10uf c18 1uf r15 1.0m c19 1uf r7 0 r8 10k r10 10k c6 10uf c14 2.2nf r13 2.2 4 1 56 5 11 42 22 55 vddc vssc vi cbp vddo vo vsso disable pip212 u5 c15 2.2nf r14 2.2 tp3 tp4 c48 22pf tp14 tp 15 tp16 tp17 tp9 r31 0 tp10 tp11 tp12 tp13 c49 22pf c43 1uf tp1 tp2 tp6 c44 1uf tp8 c27 100uf r23 10 4 1 56 5 11 42 22 55 vddc vssc vi cbp vddo vo vsso disable pip212 u6 c30 100uf c3 10uf c2 10uf vo1 (2.5v, 15a) c32 0.1uf l1 1uh refout 9 pgnd 22 gdh2 20 bst2 19 in1- 4 vpn1 27 sync 6 cs1+ 1 pvcc 23 comp1 5 ss1/en1 28 cs1- 2 ss2/en2 15 comp2 11 in2- 12 refin 10 cs2- 13 cs2+ 14 vpn2 18 vin2 17 bst1 26 gdh1 25 gdl1 24 gdl2 21 ref 8 rosc 3 agnd 7 avcc 16 u1 SC2446A c4 10uf c5 10uf c7 10uf c33 0.1uf c35 1uf c34 0.1uf r12 10k c24 100uf c22 10uf vi ngnd c25 10uf vin=12v 22 ? 2005 semtech corp. www.semtech.com power management SC2446A typical characteristics channel 2: vo2 = 1.2v channel 1: vo1 = 2.5v d ual phase th erm als vin=12v, d ual output, fsw=500kh z, tamb=25c , 0lfm 0 10 20 30 40 50 60 70 02 46 8101214161820 amps te m perature (de 2. 5v t herm als 1. 2v t herm als 2. 5v t herm als 1. 2v t herm als SC2446A + pip212 dual phase efficiency vin=12v, d ual outp ut, f sw=500kh z, tam b=25c , 0lfm 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 2 4 6 8 10 12 14 15 16 18 20 amps efficien c 2. 5v ef ficiency 1. 2v ef ficiency 23 ? 2005 semtech corp. www.semtech.com power management SC2446A ch1 (2.5v) output voltage ripple = 14.0mv p-p typical characteristics (cont.) ch2 (1.2v) output voltage ripple = 15.6mv p-p 24 ? 2005 semtech corp. www.semtech.com power management SC2446A vin = 12v io1 vout1 = 2.5v typical characteristics (cont.) vin = 12v vout1 = 2.5v ch1 (2.5v) full load start-up ch1 (2.5v) shutdown 25 ? 2005 semtech corp. www.semtech.com power management SC2446A ch2 (1.2v) full load start-up typical characteristics (cont.) ch2 (1.2v) shutdown vin = 12v io2 vout2 = 1.2v vin = 12v vout2 = 1.2v 26 ? 2005 semtech corp. www.semtech.com power management SC2446A vin = 12v, dual output, fsw = 500khz, tamb = 25 c, 0lfm 2.5v output short and release. typical characteristics (cont.) vin = 12v, dual output, fsw = 500khz, tamb = 25 c, 0lfm 1.2v output short and release. v o2 v o1 i o1 v o2 v o1 i o2 27 ? 2005 semtech corp. www.semtech.com power management SC2446A land pattern - tssop-28 semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phone: (805)498-2111 fax (805)498-3804 contact information outline drawing - tssop-28 n a a2 a1 bxn e1 .378 9.60 .386 9.70 9.80 plane bbb c a-b d ccc c dimensions "e1" and "d" do not include mold flash, protrusions 3. or gate burrs. datums and to be determined at datum plane controlling dimensions are in millimeters (angles in degrees). -b- notes: 1. 2. -a- -h- side view a b c d e h e/2 (.039) .008 - .004 .024 - - - - 0 l (l1) c 01 gage plane see detail detail a a 0.25 .026 bsc .252 bsc 28 .004 .169 .173 .007 - 28 0.10 0.65 bsc 6.40 bsc 4.40 - .177 4.30 .012 0.19 4.50 0.30 .382 2x n/2 tips seating aaa c e/2 indicator pin 1 2x 2 13 .018 .003 .031 .002 - 8 0 0.20 0.10 - 8 0.45 0.09 0.80 0.05 .030 .007 .047 .042 .006 - 0.60 (1.0) - 0.75 0.20 - - - 1.20 1.05 0.15 d reference jedec std mo-153, variation ae. 4. inches b n bbb aaa ccc 01 e1 e l l1 e d c dim a1 a2 a min max millimeters dimensions min max nom nom e (.222) (5.65) z g y p (c) 4.10 .161 0.65 .026 0.40 .016 1.55 .061 7.20 .283 x inches dimensions z p y x dim c g millimeters this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 1. |
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